Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of one or more conductive objects, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.
Another user interface device that has become more common is a touch screen. Touch screens, also known as touchscreens, touch windows, touch panels, or touchscreen panels, are transparent display overlays which are typically either pressure-sensitive (resistive or piezoelectric), electrically-sensitive (capacitive), acoustically-sensitive (surface acoustic wave (SAW)) or photo-sensitive (infra-red). The effect of such overlays allows a display to be used as an input device, removing the keyboard and/or the mouse as the primary input device for interacting with the display's content. Such displays can be attached to computers or, as terminals, to networks. Touch screens have become familiar in retail settings, on point-of-sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs where a stylus is sometimes used to manipulate the graphical user interface (GUI) and to enter data. A user can touch a touch screen or a touch-sensor pad to manipulate data. For example, a user can apply a single touch, by using a finger to touch the surface of a touch screen, to select an item from a menu.
Electronic systems that include touch screens are sensitive to a variety of noise sources. Noise coupled into touch screens and circuitry for applying and measuring the presence of a touch on a touch screen (hereinafter “touch controllers”) may result in significantly reduced accuracy, resulting in false touches and the reported location of a touch on a touch screen. FIG. 1 depicts various paths that induced noise currents may take between a source of noise, a user of a touch screen, and the touch screen and touch controller. The main sources of induced noise include (1) switching regulators employed in DC-AC, DC-DC, and AC-DC converters used in power supplies and battery chargers for touch controllers and nearby external electronic equipment; (2) electromagnetic noise emitted by external electronic equipment, for example, by high speed motors during operation of the external equipment (i.e., in vacuum cleaners, mixers, washing machines, etc); (3) noise emitted by radio transmitting equipment (e.g., radio stations, mobile communications, CB radios, etc); and (4) noise emitted by high-voltage power lines.
These noise sources may emit electromagnetic radiation over a narrow band of frequency, a wideband of frequencies, or both under different operating conditions. Of the noise sources listed above, chargers and power supplies for use with touch screens are the most common and cause the largest number of false or inaccurate touch events reported by touch controllers. FIGS. 2A-2C, for example, depict amplitude vs. time and frequency for common mode noise emitted by a battery charger under different load conditions. The number of harmonics, the fundamental frequency, and amplitude of common mode charger noise depend on charger type, charger electronics, and charger physical construction. As shown in FIGS. 2A-2C, harmonic frequencies change during the battery charge process, resulting in a frequency band of noise that changes with time but generally has a narrow bandwidth.
Radiated noise may be coupled into conductors of the touch screen and/or touch controller radio transmitting equipment. The coupled radiated noise signal waveform type depends on the environment. The waveforms may be simple sine waves or waveforms with several tens of harmonics.
Noise sources generated internally within touch controllers may include internal charge pumps, communication lines, LCDs, etc.
A touch controller's response to injected noise depends to a large degree on the frequency response characteristic of a touch screen measurement channel within the touch controller. The measurement channel frequency response of a conventional touch controller is shown in FIG. 3. The frequency response characteristic shown in FIG. 3 contains high sensitivity zones (200-250 KHz, 700-750 Khz) and low sensitivity zones to noise. If a harmonic of injected noise coincides with one or more of the high sensitivity zones, the touch controller accuracy may be significantly reduced, resulting in false touches, etc. This is demonstrated in FIG. 4, which illustrates the effects of noise having harmonic components that coincide with a measurement channel response characteristic. FIG. 4 shows the effect of harmonically-induced noise on raw counts of a simulated finger touch over time as the amplitude of the harmonically induced noise increases.